The casting of metal articles using sand molds, sand shells and sand cores is well known in the art. Detailed information regarding the state of this technology can be found, for example, in a text by James P. LaRue, EdD, Basic Metalcasting (The American Foundrymen's Society, Inc., Des Plaines, Ill., 1989). Using such a technique, a mold can be made from a mixture of sand and (typically) an organic binder by packing the mixture loosely or tightly around a pattern. The pattern is then removed, leaving a cavity in the sand which replicates the shape of the pattern. Once the organic binder is shape-stabilized by any of a number of hardening techniques (as described below), the cavities in the sand mold are filled with molten metal by pouring the molten metal into the mold.
Many traditional shaping techniques exist for forming loose masses of particulate, fibers, whiskers, etc., into a desired shape followed by some set of processing conditions which typically involve high temperature exposures. For example, many traditional ceramic processing techniques such as slip casting, dry pressing, isostatic pressing, hot pressing, extrusion, etc., each involves the consolidation of an initial loose mass or unbonded array of constituents into a shaped member having at least some structural integrity. Moreover, in each of these techniques some means for initially holding the loose mass together until the loose mass can itself consolidate into a preferred shape is necessary. Common to many of the traditional approaches is the use of a binder system which imparts at least some initial "green" strength to the body to permit the body to hold its predetermined shape.
Further, common to each of the aforementioned traditional techniques is the application of thermal energy. A primary purpose of the application of thermal energy is to permit individual constituents of the green body to begin to, for example, sinter together to form a more rigid body. Typically, when such sintering occurs, a part will change in size and/or shape due to porosity in the green body being consolidated. It is during such sintering operations that cracking, bending, and/or uncontrolled shrinking may occur. The art is replete with many techniques for controlling undesirable aspects associated with traditional sintering processes.
The art also includes processing techniques for the formation of composite bodies. For example, rather than starting with any of the constituents discussed above and causing such constituents to consolidate into a dense, shaped body, the art teaches that porosity in a first material can be filled with a second material to form a desirable composite body. For example, the porosity in a first formed body could be filled with an inorganic material such as a ceramic or a glass, a polymer, a metal or alloy, an intermetallic and the like. The impetus for forming a composite body is to achieve a synergistic interaction between the constituents of the composite. Specifically, a single material by itself may not be able to withstand certain corrosive and/or erosive environments and/or certain high temperature environments, etc. However, by combining two or more materials together, desirable attributes of both materials may be utilized to overcome the shortcomings of a single material.
A key element for reliably and economically producing desirable composite materials involves the ability to produce economically and reliably a shaped first material into which a second material can be introduced. Many techniques exist for shaping a porous first material into an acceptable body for introduction of a second material or matrix therein; however, the search continues for better techniques to form porous first materials. This invention attempts to satisfy the need for achieving a reliably and economically produced first material which reliably and economically accepts a second material to result in a desirable composite body.
Binders used for the preparation of preforms for use in the fabrication of metal matrix and ceramic matrix composites are typically wholly organic, or wholly inorganic compositions. Organic binder systems which may perform well under certain processing conditions may otherwise suffer certain disadvantages. For instance, in the fabrication of aluminum oxide matrix, ceramic matrix composites or aluminum matrix, metal matrix composites it is preferable to have a minimum carbon residue in the finished composite. Thus, it is essential that organic binders used in the formation of preforms are substantially completely burned out prior to infiltration of the metal matrix or growth of the ceramic matrix. The resulting preform is often weak and requires careful handling prior to the matrix introduction. When wholly inorganic binders are incorporated in the fabrication of preforms, undesirable inorganic phases may result within the final composition, such as, for example, silicon dioxide, which can lower the thermal performance of these composites.
Organometallic, ceramic precursors are known in the art of ceramic processing. These materials can be in the form of either solvent-soluble solids, meltable solids, or hardenable liquids, all of which permit the processibility of their organic counterparts in the fabrication of ceramic "green bodies". During the sintering of such green parts, however, the ceramic precursor binders have the added advantage of contributing to the overall ceramic content of the finished part, because the thermal decomposition of such ceramic precursor binders results in relatively high yields of ceramic "char". Thus, most of the precursor is retained in the finished part as ceramic material, and very little mass is evolved as undesirable volatiles. This second feature is advantageous, for example, in reducing part shrinkage and the amount of voids present in the fired part, thereby reducing the number of critically sized flaws which have been shown to result in strength degradation of formed bodies.
Such precursors can be monomeric, oligomeric, or polymeric and can be characterized generally by their processing flexibility and high char yields of ceramic material upon thermal decomposition (i.e. pyrolysis). These precursors are neither wholly inorganic nor wholly organic materials, since they comprise metal-carbon bonds. These precursors can be distinguished from other known inorganic binders for sand mold fabrication described above (which comprise no carbon), and other known organic binders (which comprise no metallic elements). It has been unexpectedly discovered that such organometallic "hybrids" which are hardenable liquids are uniquely suited for use as binders for sand grains in the fabrication of sand molds, cores, and shells, since they can provide excellent mold strength at extremely low binder levels. Their utility resides in a unique combination of, for example, the processing flexibility afforded by organic binders and the high char forming characteristics and improved adhesion to sand of inorganic binders. Such binders can therefore be easily processed to provide a hardened sand mold, and subsequently used for metalcasting with a minimum of toxic volatiles being evolved.
Further, it has been unexpectedly discovered that such organometallic "hybrids" are uniquely suited for use as binders for filler materials in the fabrication of preforms to be used in the formation of composite materials. For example, such organometallic "hybrids" have been found to be uniquely suited to the formation of metal matrix composites by molten metal infiltration processes (e.g., spontaneous infiltration, pressure and vacuum assisted infiltration, etc.). Moreover, these organometallic "hybrids" have also been found to be useful as preform binders for ceramic matrix composite formation processes (e.g., directed metal oxidation, sintering, isostatic pressing, chemical vapor infiltration, etc.). Additionally, organometallic "hybrids" have also been found to be useful as preform binders for polymer matrix composite formulation processes. Further, since such organometallic, ceramic precursor binders are also liquids, they can be employed directly without use of a solvent. This obviates the emissions and disposal problems associated with solvent-based systems which require a "drying" step subsequent to mold shaping. Further, traditional binder materials "burn-out" when heated, yielding performs with little or inadequate strength. Conversely, the binders of the present invention "burn-in", that is, the binders of the present invention may be converted in high yield to a ceramic when heated, thus yielding preforms with excellent strength for subsequent composite formation processes.
Siloxanes have been used in the past to improve the adhesion of such binder systems as polycyanoacrylates to sand grains (see, for example, U.S. Pat. No. 4,076,685). In such a system the siloxane is used as a processing aid rather than the binder itself. Additionally, partial condensates of trisilanols have been used in combination with silica as binder systems which are provided in aliphatic alcohol-water cosolvent (see, for example, U.S. Pat. No. 3,898,090). Such in-solvent binders have been shown to suffer the disadvantage of short shelf life ("several days") due to additional silanol condensation during storage. A further disadvantage is that these binders require the step of solvent removal from the core or mold by a drying process ("to remove a major portion of the alcohol-water cosolvent") before metalcasting. Otherwise, voids and poor mold integrity result during the metalcasting process. The use of hardenable, liquid organometallic, ceramic precursors as solventless binders for the fabrication of sand molds, shells, cores, and binders for preforms has not been disclosed.